1. Field of the Invention
The present invention relates to a sheet feeding device having a mechanism for blowing air to a placed sheet stack and floating and loosening the sheets.
2. Description of the Related Art
Conventionally, in sheet feeding devices provided in image forming apparatuses such as printers and copying machines, an air loosening mechanism which blows air to an end of sheet stack and floats and loosens the sheets is provided in order to securely feed sheets one by one from the sheet stack placed in a containing portion. As shown in FIG. 9, with the air loosening mechanism, air is blown to the end of the sheet stack contained in a containing portion 100 from a loosening nozzle 101 and a separation nozzle 102, a plurality of upper sheets are floated and loosened. The floated top sheet is stuck to a conveying belt 103 and is simultaneously conveyed so that the sheets are separated one by one. The separated sheets are further conveyed to a downstream side by a drawing-out roller pair 104 provided on a downstream side. The device having such a constitution is disclosed in, for example, Japanese Patent Application Laid-Open No. 7-196187.
An auxiliary air loosening mechanism, which blows air to a side of placed sheet stack (a direction perpendicular to a sheet feeding direction) and floats and loosens the sheets, is added so that also sheets of large size can be securely floated and loosened. As shown in FIG. 9, in this mechanism, auxiliary separating fans 105 and 106 are provided on a side of sheet stack, and air is blown to the side surface of the sheet stack from openings 105A and 106A, so that the sheets can be securely floated. The device having such a constitution is disclosed in, for example, Japanese Patent Application Laid-Open No. 2003-182873.
In the case where sheets to be fed are so-called coated paper which is used for printing and whose surface is coated with a coating material, the sheets are possibly stuck to each other. The sticking power of the sheets (the power of sticking the sheets) occasionally becomes 1 kgf or more according to temperature and humidity of a use environment. In such a case, two sheets which are stuck to each other are fed and also occasionally ten or more sheets which are stuck to each other are fed, thereby causing paper jam. In order to float heavy and large sheets whose basis weight is 200 g/m2 or more, even if an influence of the sticking is not present, very strong wind pressure is required only for floating.
In the case where a device, which can output about 70 to 100 sheets of A4 size per minute, is assumed, loosening time per sheet reduces, and a sheet feeding condition becomes further strict.
For example, in the case where it is assumed that about 50 coated sheets of A3 size whose basis weight is 200 g/m2 in an environment such that room temperature is 30° and relative humidity is 60 to 80% are output perminute, it is found according to an experiment that about 650 Pa of the wind pressure of air to be blown in order to securely float the sheets is necessary.
In centrifugal fans such as sirocco fans which are used in copying machines or the like being capable of outputting 50 to 70 sheets of A4 size per minute, comparatively large fans have a diameter of impeller which is about 80 mm to 120 mm. Such fans can obtain air whose pressure is higher than that of axial fans with the same diameter.
However, even in the case where a sirocco fan having impeller with a diameter of 120 mm is used, only the wind pressure of about 420 Pa is obtained.
In order to generate high-pressure air, a compressor or a large-sized centrifugal fan (for example, a turbo fan or a sirocco fan) is occasionally used, but devices mostly become large, heavy and expensive in all the cases. For this reason, a sheet feeding device becomes large and expensive.